The present invention relates to a rotary die cutter for blanking corrugated fiberboard sheets or the like.
As a rotary die cutter of this type, a cutter, for example, shown in FIGS. 4 to 6 has been publicly known. In this cutter shown in FIG. 4, frames 1 are erected on both sides, right and left, and a die cut cylinder 3 and an anvil cylinder 4 are rotatably supported on these frames 1 via bearings 5.
The die cut cylinder 3 has a cylindrical outer peripheral surface, and at opposite ends thereof are provided support shafts 3a coaxially. Each of the support shafts 3a is rotatably supported on the frame 1 via the bearing 5. Similarly, the anvil cylinder 4 also has a cylindrical outer peripheral surface, and at opposite ends thereof are provided support shafts 4a coaxially. Each of the support shafts 4a is rotatably supported on the frame 1 via the bearing 5.
At a portion where each support shaft 3a protrudes from the frame 1, a gear 6 is provided to cause the die cut cylinder 3 to rotate. Also, at a portion where each support shaft 4a protrudes from the frame 1, a gear 7 is provided to cause the anvil cylinder 4 to rotate. These gears 6 and 7 mesh with each other.
Each frame 1 is provided with a side cover 8 for covering the gears 6 and 7. This side cover 8 is constructed such that a portion under the gears 6 and 7 (a portion under the bearings 5) forms an oil reservoir 8a for storing a lubricating oil. The lubricating oil stored in the oil reservoir 8a adheres to a gear 10 supported rotatably on the frame 1, and further adheres to the gear 7 meshing with the gear 10, so that the gears 7 and 6 are lubricated. The bearing 5 is lubricated by grease loaded inside.
As shown in FIG. 6, a blanking section itself is constructed so that the rotation is transmitted from, for example, a printing unit (not shown) on the upstream side by a connection gear 13 meshing with the gear 6, and is further transmitted to a unit (not shown) on the downstream side by a connection gear 14 meshing with the gear 6.
As shown in FIGS. 4 and 5, a blanking die 11 is installed on the die cut cylinder 3, and this blanking die 11 is provided with knives 11a. The knife 11a performs blanking of a corrugated fiberboard sheet 12 by holding the corrugated fiberboard sheet 12 between the knife 11a and the surface of the anvil cylinder 4.
The rotary die cutters come in two types: a cutting type called soft cut in which a sawtooth knife is used as the knife 11a and a rubber anvil is used as the anvil cylinder 4, and a cutting type called hard cut in which a straight tooth knife is used as the knife 11a and a metal anvil is used as the anvil cylinder 4.
In the case of the latter hard cut, since cutting is performed by pressing the hard knife 11a on the hard anvil cylinder 4, the pressing amount, in other words, a center distance between the die cut cylinder 3 and the anvil cylinder 4 is required to be maintained precisely (usually, an accuracy in the order of 1/100 mm is strictly kept). If such a center distance cannot be kept, an uncut portion remains on the corrugated fiberboard sheet 12, or the blade edge of the knife 11a is collapsed by an excessive pressure.
In the above-described rotary die cutter, the bearings 5 are heated by the rotation of the die cut cylinder 3 and the anvil cylinder 4, and part of this heat is transmitted to the frames 1, so that the temperature of the frames 1 rises. Therefore, the frame 1 is elongated by thermal expansion, resulting in an increase in the distance between the support shafts 3a and 4a of the die cut cylinder 3 and the anvil cylinder 4.
For this reason, a cutting pressure of the knife 11a acting on the anvil cylinder 4 decreases, or sometimes a gap is produced between the knife 11a and the anvil cylinder 4, which leads to a possibility that improper cutting occurs. To overcome this problem, with the elapse of time, it is necessary to adjust the aforesaid distance between the support shafts 3a and 4a, or to adjust the blanking die 11, which causes the hindrance to productivity.
Also, part of the aforesaid heat is transmitted to the die cut cylinder 3 and the anvil cylinder 4 via the support shafts 3a and 4a, respectively, by which the temperature of the cylinders 3 and 4 are also increased. At this time, after the support shafts 3a and 4a first becomes hot, the heat conducts gradually from both the ends toward the central portion of the cylinders 3 and 4. That is to say, in each cylinder 3, 4, a temperature gradient is created from both the ends to the central portion. Thus, as indicated by the dashed line in FIG. 4, each cylinder 3, 4 is thermally deformed into a concave form. In this case, a gap between the knife 11a and the anvil cylinder 4 is narrow at portions at both ends of each cylinder 3, 4, and a gap between the knife 11a and the anvil cylinder 4 is wide at the central portion thereof, so that the cutting conditions becomes nonuniform in the width direction.
Further, since the amount of temperature rises of the frames 1 and the cylinders 3 and 4 are not equal, there is a difference between the change amount of center distance between the cylinders 3 and 4 caused by the thermal expansion of the frames 1 and the change amount (average amount in the width direction) of outside diameter of each cylinder 3, 4 caused by the thermal expansion of each cylinder 3, 4 itself. Therefore, with the elapse of operation time, the pressing pressure (average pressure in the width direction) of the knife 11a on the anvil cylinder 4 changes undesirably, which also makes the cutting conditions nonuniform.